Bifocal contact lens having an optical zone defined by an aspheric posterior curve and a spherical anterior curve are known. The aspheric back curve portion has a controlled degree of change so that, within a predetermined power range, light rays from objects at all distances focus directly on a common focal point. When placed on a human cornea, this contact lens acts as a refractive medium to focus light rays back upon the retina.
The above lens is a multi-focal design with the spherical front curve directly relating to the aspheric optical zone back curve. However, since the aspherical back curve entirely dominates the optical zone, distance night vision becomes impaired. This is because when the eye pupil dilates at night it no longer predominantly receives light through the center region of the lens and instead tends to pick up light further into the aspheric part of the lens which is normally used for near vision.
It is accordingly one object of the present invention to provide a contact lens having aspherical curve characteristics to maintain the advantage of a multi-focal design while providing curvature characteristics, within the optical zone portion of the lens, which improves night vision.
Another object of the invention is to improve night vision by providing a distant vision zone in the center of the optical zone, in the form of a spherical base curve.
Still another object is to provide the foregoing curvature characteristics in a multi-focal soft contact lens.
FIG. 1 is an illustration of a tool 10 used for cutting an aspheric posterior curve in a pre-cut and polished lens blank 12. Tool 10 comprises a lens holder 14 having a cylindrical sidewall 16 formed with internal threads 17 and a transverse front end wall 18 having a central opening 19 receiving the pre-cut lens blank 12 therein. The lens blank body 12 has a diameter corresponding to the diameter of the front end wall opening 19 through which it extends. The lens blank body 12 is formed with an annular flange 20 of a larger diameter which is received against the rear facing surface 22 of the end wall 18 in seating contact thereagainst. A cylindrical retainer 24 having an external thread 25 along its cylindrical sidewall has a central depression 26 in its transverse front end receiving a ball 27 adapted to apply pressure to the center of the lens blank body 12 upon threading of the retainer into the lens holder. In such threaded contact, a radial set screw 28 extending through the cylindrical sidewall 16 of the lens blank holder 14 is rotated to contact and apply a radial tightening pressure to the retainer 24 to tighten the parts 14,24 together. The rear end of the retainer is formed with a rearwardly extending cylindrical extension 29 adapted to be received in the chuck of a lathe in preparation for cutting the front facing surface of the lens blank 12 projecting from the mounting tool 10.
There are a number of problems associated with the prior art device of FIG. 1. For example, once a desired amount of pressure was exerted against the lens 12 by the ball 27, tightening of the set screw 28 to clamp the two pieces 14,24 together tended to shift the cutting axis C of the lens blank in relation to the mounting axis M of the retainer in the collet or chuck. This misalignment tended to cause the lens to be cut off-center. Notwithstanding possible misalignment caused by tightening of the parts 14,24 together with a radial set screw 28, there also existed a tendency for the lens not to run perfectly true in the lathe by virtue of the two part construction wherein the mounting axis M was formed in one part 24 while the cutting axis C was formed in the front or lens blank holder part 14. In other words, the inherent nature of the two-part construction required extremely precise machining tolerances to enable proper alignment of the cutting axis C with the mounting axis M.
It is another object of the present invention to provide a tool for cutting a contact lens wherein the cutting axis is properly aligned with a collet mounting axis of the tool.
Yet another object is to provide a tool for producing a desired distortion in a lens blank body in controlled, easily measured amounts.
Yet another object is to be able to distort the lens blank to permit formation of aspheric surfaces therein in preselected areas of the lens blank.
Another object is to permit distortion of the lens blank in certain areas of the concave surface without distorting other preselected areas of the concave surface to permit the formation of aspheric and spheric surfaces in the lens blank.
Yet another object is to provide a tool for cutting a lens blank wherein the lens cutting axis and the mounting axis are formed in the same component of the tool.